1. Field of the Invention
The present invention relates to a high-frequency surgical instrument for the monopolar cutting and coagulating of biological tissues by means of a high-frequency electric current, whereby a neutral electrode is electrically connected across a capacitor against ground potential that the patient conducts the lowest possible high-frequency electric voltage against ground potential.
2. Discussion of the Prior Art
By definition, high-frequency surgical instruments used for monopolar surgical techniques are equipped in each case with an active electrode and an inactive, so-called neutral electrode. During the whole duration of the surgery, the neutral electrode is electrically connected to a large area of the skin of the patient, for example, to the thigh or the upper arm.
In principle, the neutral electrode can as a rule be electrically grounded as close as possible to the grounded operating table or may be insulated against ground potential. During the activation of the high-frequency surgical instrument, the grounded neutral electrode prevents the conduction of a high-frequency current through parts of the patient's body which are in contact with objects that are directly or capacitively grounded. Furthermore, electronic instruments, especially measuring and monitoring devices could be disturbed by the high-frequency alternating voltage which the patient would in that case conduct.
The grounding of the neutral electrode and related grounding of the patient create a danger that electric current from other grounded current sources, for example, defective electric appliances which simultaneously touch the patient in an electrically conductive manner, may flow through the patient and endanger him.
As a compromise between electrical grounding and the best possible insulation of the neutral electrode against ground potential, there are known high-frequency surgical instruments in which the neutral electrode is grounded across a capacitor. As a result, given suitable dimensioning of the capacitor, the neutral electrode lies, as regards the high-frequency voltage, practically at ground potential while low-frequency electric currents through the patient or the neutral electrode against ground potential remain sufficiently small. In the safety regulations for high-frequency surgical instruments VDE 0750, part 202, the capacity of this capacitor is limited to a maximum of 50 nF.
Regarding the safety of the patient with respect to risks from low-frequency electric currents, the recommendation of the IEC (International Electrotechnical Commission) Publication 601-1 subdivides electromedical instruments into three types according to the maximum permitted low-frequency leakage current, that is, the types B, BF and CF. In accordance with IEC 601-202 (identical with VDE 0750 part 202), high-frequency surgical instruments must conform to type BF or type CF. On instruments of type BF, the patient leakage current may reach a maximum of 0.1 mA on normal function and a maximum of 0.5 mA in the first error function. On instruments of the type CF, this leakage current may attain a maximum of only one tenth of the limiting values of the type BF. Furthermore, on instruments of the type CF, the leakage current which may flow through the patient into the high-frequency surgical instrument when the patient comes in contact with grounded line voltage must not exceed 0.05 mA.